Why is kilogram’s definition changed?

NP NEWS 24 ONLINE – The way we decide the mass of everything on Earth is wrong. The end is nigh for the kilogram as we know it, or at least how metrologists – scientists who measure things for a living – know it.

Later today, representatives of the 60-metre convention states will meet in Versailles, France, and vote on whether we should bid a fond farewell to the old SI base units’ definitions. The overhaul will do away with the kilogram artifact, the last remaining object to define a unit and introduce a metric system described in terms of universal natural constants.

The new definition

Metrologists decided to base the SI units’ new definitions on the most stable thing in the universe: natural constants. The kilogram will be described by the Planck constant, h (6.62×10−34m2 kg/s), a quantum mechanical quantity that, in a wider sense, relates mass to energy via E = mc2.

Will this change how heavy a kilogram is?

No. The world will still weigh the same (6×1024kg), whether expressed in the old or new kilogram. Only what exactly we mean by 1kg or 1mol will change.

So does it make any difference in practice?

No, apart from in some special cases, for example, high-temperature measurements.

As long as the Kelvin is tied to the triple-point of water at 273.16K (0.01°C), measuring very cold (below 20K) and very hot (above 1300K) things is tricky. Using regular standardized thermometers is out of the question.

Why?

The key thing was to give the natural constants – h, NA, k and e – fixed numerical values. For this, they need to be measured as accurately as possible; otherwise, an old kilogram might end up lighter or heavier than a new one.

Scientists around the world have carried out sophisticated experiments, like the 1kg pure silicon sphere created to calculate NA and h. A strong competitor for the most perfectly round object in existence, if the sphere were the size of the Earth, the top of its highest mountain and the floor of its deepest valley would be separated by just 10m. The Avogadro sphere team managed to measure NA to an uncertainty of 12 parts per billion.